Licenses: introduce SPDX Unique Lincense Identifiers

Like many other projects, U-Boot has a tradition of including big
blocks of License headers in all files.  This not only blows up the
source code with mostly redundant information, but also makes it very
difficult to generate License Clearing Reports.  An additional problem
is that even the same lincenses are referred to by a number of
slightly varying text blocks (full, abbreviated, different
indentation, line wrapping and/or white space, with obsolete address
information, ...) which makes automatic processing a nightmare.

To make this easier, such license headers in the source files will be
replaced with a single line reference to Unique Lincense Identifiers
as defined by the Linux Foundation's SPDX project [1].  For example,
in a source file the full "GPL v2.0 or later" header text will be
replaced by a single line:

        SPDX-License-Identifier:        GPL-2.0+

We use the SPDX Unique Lincense Identifiers here; these are available
at [2].

Note:...